-ddump-splices does not parenthesize rank-n contexts correctly

Ryan discovers another -ddump-splices bug—news at 11:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

$([d| f :: forall a. (Show a => a) -> a
      f _ = undefined
    |])

$ /opt/ghc/8.4.3/bin/ghci Bug.hs -dsuppress-uniques
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help                                                                                                                                             
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(6,3)-(8,6): Splicing declarations
    [d| f :: forall a. (Show a => a) -> a
        f _ = undefined |]
  ======>
    f :: forall a. Show a => a -> a
    f _ = undefined

Notice that f's type signature is pretty-printed as forall a. Show a => a -> a, which is just wrong. Patch incoming.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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